1. Field of the Invention
The present invention relates in general to electric motors and, in particular, to a system in which a single controller is used to control two or more motors. This invention has particular application in HVAC systems.
2. Background Art
As is well-known in the HVAC art, furnaces include a main blower that forces the heated (or cooled) air into the supply duct for distribution throughout a zone (or zones) to be conditioned. Recently, furnaces have been additionally equipped with a second, smaller blower, called an induced draft blower, which draws exhaust gasses out of the furnace. This induced draft blower is matched to the main blower such that these two blowers work in combination to provide efficient conditioning of a particular space.
To further increase efficiency of the HVAC system, it is desirable to run a furnace at various rates (by varying gas supply and motor speed). Traditionally, furnaces were controlled such that they would continue to distribute conditioned air (i.e. heated or cooled) to the desired zone or zones at 100% power, intentionally overshooting the desired temperature, and then shutting down completely until the conditioned zone fell significantly below the desired temperature after which the zone was heated again at 100% power. It has been found, however, that a desired temperature can be more efficiently reached and then maintained by running the furnace at 100% when the temperature of a zone is significantly below the desired temperature and then at 25-60% of its maximum power over a longer period of time to maintain the zone at or about desired temperature. This approach can result in a significant cost-savings to the user, while also providing increased comfort.
This lower power mode allows for and requires a reduced gas input and slower rotation of both the main (circulating) and induced draft blowers to maintain proper furnace operation. This desired adjustability can be easily achieved with today's modern speed controllers, which are available for most motors. Necessarily, however, each controller adds to the cost of its associated motor and, in turn, the overall system into which the motors are installed. Accordingly, it is an object of the present invention to provide a control scheme in which a plurality of associated motors can be controlled by a single controller.
One very popular type of motor in furnaces is the brushless DC permanent magnet motor. Because of its brushless design, these motors require some form of feedback to their associated controller. Normally, this requirement is no problem. However, designing a single controller to accept and combine the feedback of differently-rated DC motors and, in turn, control both of those motors with a common signal would be difficult, at best. It is believed that in such a circuit, drive speed could vary by more than 20%. Thus, standard furnaces, which use brushless DC motors, may present difficulties toward achieving the goal of running a plurality of motors from a single controller.
Accordingly, it is an associated object of the present invention to select a type of motor for use in furnaces that will facilitate the desired objects of the present invention.
These and other objects of the present invention will become apparent to those of ordinary skill in the art in light of the present specification, drawings and claims.